1. Field of the Invention
The present invention relates to decontamination of radio-contaminated metals, and in particular to decontamination of radio-contaminated metals by reductive electrochemical processing. Of particular interest to the present invention is the remediation of radio-contaminated nickel from decommissioning uranium gas diffusion cascades in which nickel is the primary constituent. However, the decontamination art taught herein applies equally well to the recovery and decontamination of other multivalent, strategic metals which can be electrowon such as copper, cobalt, chromium, iron, zinc and like transition metals.
2. The Prior Art
The radiochemical decontamination art is presented with unique practical problems not shared with traditional extraction technologies. Radiochemical extraction technologies are generally concerned with the economic recovery of "product radiochemicals". Routine process inefficiencies which permit residual amounts of radiochemicals to remain in process streams or in by-products raise only normal economic issues of process yield and acceptable process costs. The various process streams and the product radiochemicals are used and will continue to be held by the regulated nuclear community so that deminimus release to the general public is not a concern. In stark contrast with these extraction technologies, the presence of only residual parts per million concentrations of fission daughter products such as technetium in remediated nickel and other like recycled products will so degrade product quality of remediated products that their release to unregulated non-nuclear markets is prevented. Degraded product must then either be employed in less valuable regulated nuclear markets or be reworked at great financial cost.
The sources of radio-contamination in diffusion barrier nickel in particular include uranium with enrichment levels above natural levels (usually about 0.7%) and reactor fission daughter products, such as Tc, Np, Pu, and any other actinides. For example, contaminated nickel may have an activity due to technetium of up to about 5000 Bq/gm or more, which is at least an order of magnitude above the maximum international release criteria of 74 Bq/gm metal total activity. Certain countries have specified an even lower criteria of 1.0 Bq/gm or less total activity. If the total activity of a metal exceeds the release criteria, then it is subject to government control for the protection of the public.
Various decontamination processes are known in the art, and specifically for decontamination of nickel. Nickel can be removed by selectively stripping from an acidic solution by electrowinning. See U.S. Pat. No. 3,853,725. Nickel may also be removed by liquid-liquid extraction or solvent extraction. See U.S. Pat. Nos. 4,162,296 and 4,196,076. Further, various phosphate type compounds have been used in the removal of nickel. See U.S. Pat. Nos. 4,162,296; 4,624,703; 4,718,996; 4,528,165 and 4,808,034.
It is known that metallic nickel, contaminated with fission products, can be decontaminated to remove any actinides present by direct electrochemical processing based on the differences in reduction potential in the electromotive force (emf) series. Actinide removal is favored by two phenomena during electrochemical plating. Actinides have a significantly higher reduction potential relative to nickel and they are normally won from molten salt electrolyte rather than from aqueous electrolyte. See U.S. Pat. Nos. 3,928,153 and 3,891,741, for example. Other electrolytic processes are disclosed by U.S. Pat. Nos. 3,915,828; 4,011,151; 4,146,438; 4,401,532; 4,481,089; 4,537,666; 4,615,776 and 4,792,385.
While the removal. of uranium and other actinides has been generally addressed by electrorefining, the removal of technetium has continued to be a substantial problem. When nickel is refined by standard art in a sulfate electrolyte solution, the technetium had been found to track the nickel and codeposit on the cell cathode. Thus, e.g., experiments employing aqueous sulfuric acid solutions at a pH of 2-4 at room temperature have shown that the technetium activity of the deposited metal may be as high as the technetium activity of the feedstock. Thus, e.g., product activity levels as high as about 24,000 Bq/gm may result from electrorefining feedstocks with initial activity levels of the order of about 4000 Bq/gm.
Accordingly, there remains a need for an economical and efficient method to decontaminate metals and more specifically, to separate technetium from these metals in a simple manner.